Avoiding Frost & Fog on Windshields
January, that magical time of year when those of us in "Winter Cities" live in
a beautiful world of white. The problem is... when that whiteness includes our car
windows it can be downright dangerous. I watched my van drive up today after being
loaned out for driving to a winter youth camp for the weekend. Some windows were
quite white. Admittedly it was a cold day of -20 C (-4 F), and I'm sure the
van was full of teenagers who just wouldn't stop breathing! However, it occurred to
me that even in Winnipeg where this is a fact of life, there are probably many people
who could use some practical advice about how their choices affect the fogging up
of a vehicle's windows.
Dew Point, Relative Humidity, and Fog in Air
Before explaining fog and frost on a window, we'll begin with water in the
air. After all, that's where the frost is coming from!
At any given air temperature there is a maximum pressure of water that air
can hold. The higher the temperature, the higher that pressure is... doubling
approximately every 10oC (20oF). So, air at room
temperature can hold approximately 4 times as much water as air at freezing
(0oC or 32oF). By 100oC
(212oF) that pressure is (sea level) atmospheric pressure. Trying to heat water
so its vapor pressure is above the pressure of the surrounding air will cause
"boiling". Below sea level (higher surrounding air pressure) water can be heated
above 100oC. Above sea level (lower surrounding air pressure), water will boil at
less than 100oC.
The ratio of the water pressure in the air compared to the maximum possible
pressure at that temperature is known as the "relative humidity" of the air.
When air has reached 100% relative humidity we say it is saturated.
If more water is put in, the air is supersaturated - which means the water will form
into droplets. For an example, look carefully at a boiling pot of water.
The water which has heated into a gas - "steam" - leaves the pot at 100oC
is invisible. However, the air above the pot is at room temperature. As the steam
mixes into the surrounding air it cools and there is now too much water gas for
the temperature it is at. So, shortly above the pot the air supersaturates, and the
water forms into tiny droplets (making a fog which many people mistakenly refer to as
the "steam"). Follow that mist further into the room, and it disappears again. Now as
the droplets get into air that isn't saturated yet there is room for more water. So
the droplets evaporate, and the relative humidity of the whole room rises. Eventually,
especially with windows closed, the whole room will supersaturate and fill with a mist,
or you will see fogging on colder surface (such as, perhaps, the window if it is
We can turn this around the other way then -- given that we have air which has
some amount of water in it, there is a certain temperature at which it will
supersaturate - this is called the dew point. Typically during the day water evaporates
into the air, but not all the way to 100% relative humidity. There is then a temperature
where the water will start to condense out of the air -
perhaps as a fog, dew, frost... this is called the
dew point. If the overnight temperature dips below this, there will be dew for the morning.
Another well known effect is how winter is "static" season due to the dry air in buildings.
The most saturated outside winter air contains very little water compared to
the saturation level for warm inside air. When the outside air comes into
your house and warms up (and if you don't eventually die of suffocation, it
means you are exchanging air with the outside) - that small water content at a higher
temperature now corresponds to a very low relative humidity.
Molecular Dynamics of Fogging and Unfogging
Let's consider this at a slightly more advanced level. Suppose you have a bowl of
water with air above it at the same temperature. There are continually water molecules
evaporating from the bowl. The higher the temperature of the water the faster this
evaporation will occur. Simultaneously there are water molecules from the air hitting
the water surface and "condensing" into the bowl. When the relative humidity is less than
100%, what is happening is that the rate water leaves the bowl is greater than the rate it
is condensing back in. At 100% relative humidity the two rates are equal so the amount
of water doesn't change. If the air supersaturates, anywhere water starts to form it will
keep growing larger and larger. Hence fog (clouds), dew and frost will form as mentioned
above. These are all indications of condensation being faster than evaporation.
This partly explains why humid hot weather is so much more upleasant than dry hot
weather. Normally in hot weather your body sweats so it can cool itself by evaporation.
The more humid the air is the slower the evaporation will be. So, there is a greater
build up of sweat - making you feel sticky, and the sweat is less effective at cooling you
(causing the body to produce more).
Now, about that car...
Inside a car there are several competing effects.
Firstly, you have at least one person breathing. The air that you exhale is very moist.
In cool weather your breath will often supersaturate as it cools to the temperature of the surrounding
air, hence you see a fog as you exhale. Every breathing person in the car is adding
water to the air. Eventually the air will saturate. You will get fog or frost. So:
job #1 is keep a good flow of air coming in from the outside. In fact,
the colder it is outside the drier that air is, so the more beneficial it is to
compensate for your breathing.
Typically a car fan will have settings from "off" (obviously not good) through various
levels up to "recirculate". The most common mistake is to drive with the fan set at
"recirculate" in order to have a high interior temperature to avoid fogging the windows.
This may work in a car, it is less likely in a van, but in either case if you park
on the street be prepared for a huge amount of frost on the inside windows when you return.
"Recirculate" means reuse the air inside the car - so yes it is faster to get to higher
temperatures, but the relative humidity of the air in the car will keep rising.
I use recirculate only to attain a more comfortable state quickly, and possibly to
melt existing frost more quickly, but to keep it on once the car temperature has started
to rise is asking for trouble.
#2: keep the car cool
Even a few degrees can affect the maximum amount of water in the air quite
significantly. The less water in the air, the slower the windows will frost up.
The dilemma here is that if the windows are frosty it seems best to heat up the car.
Its best to scrape that frost off, but if you want to do it with heat take a long enough
drive that you can turn down the temperature and drive until the air is cool and dry
inside (or else you'll have the same start with frosty windows problem next time).
#3: Use the temperature setting to control the temperature
That might seem obvious, but I often see people turn down the airflow as the
car gets too hot. It might be more pleasant due to less wind and reducing noise in the car,
but if frosting windows is your concern don't lose sight of #1 above. There is a
temperature control that affects how the air is heated as it enters the car - use that
to control the car temperature.
#4: warm the windows
Your windows will be colder than the air inside the car. This is especially true
if cold rain or snow is hitting them, but with a constant flow of cold air
against them as you drive they will naturally cool down. You need to direct
the warm air from the vents against the windows to compensate for this. The bigger the
difference between the air and the window temperatures, the
lower the relative humidity that will still fog the windows up.
This can be illustrated with the doubling every 10oC rule. For example, suppose
the air is 20oC (68oF), and a window is at 10oC
(50oF). If the air in the car is over 50% relative humidity the window
will fog up. If the window is at 0oC (32oF) it will
fog up if the air is above 25% relative humidity. Hence, warming the window by even
a few degrees can make all the difference between a window fogging up or not under given
#5: keep outside air flowing over the windows
#4 emphasized using warm air flow against the windows. Even if the air
isn't warm, the fact is that
outside air - having been colder - will probably be drier than the air inside the
car. The drier the air against the window the faster the water can evaporate off
because there is less condensing back on from the air. You can demonstrate this
by putting a little water in two saucers. Leave one standing, set a fan blowing over
the other. The one with the fan will dry out faster (both will take a long time,
depending on how much you put in). The one without the fan gets
moist air over the water surface from evaporation, and without that being
pushed away it is soon condensing back in about as fast as it evaporates.
Something curious I've found is that fogging windows isn't exclusively
a winter phenomena. We sometimes get the windows fog up on hot humid days
and long trips. With the air conditioner running full time in the car the
windows can become enough cold compared to the outside air to start fogging up
on the outside. This only happens significantly near air vents, so an obvious solution
is to change the vent direction. If it happens on the front windshield there are
always the wipers to help out. Failing all else... it might be necessary to turn off
the air conditioning for a while.
What about anti-fog coating?
These are popular for glasses, I don't know if I've ever seen them advertised
for windows. What I've written about above talks about water in the air and on
surfaces, but it is possible to slow the buildup of water on surfaces in the first
place to reduce the problem. That's what antifog coatings are for.
Maybe I'll write about that another time...
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